1. Field of the Invention
The present invention relates to an X-ray digital tomographic image taking apparatus which visualizes a tomographic image of a subject based on X-rays that have passed through the subject.
2. Relate Background Art
An X-ray tomographic image taking method using an X-ray film is being conventionally practiced. This X-ray tomographic image taking method irradiates a subject with X-ray a plurality of times while moving an X-ray tube and X-ray film synchronously in the directions opposite to each other with respect to the subject placed between them in such a way that the center of the X-ray beam aligns with the center of the X-ray film, in order to multiple expose the respective X-ray transmitted images on a single film and thereby obtain a tomographic image with blurred images of components other than those located in the vicinity of the tomographic image of the subject. Furthermore, a digital tomographic image taking apparatus has also been commercialized which obtains digital image data using an I.I. (image intensifier) instead of an X-ray film and a TV image pickup system and there reconstructs the image by adding up the data to thereby obtain the identical tomographic image as an image of multiple exposed images on a film.
On the other hand, a dual energy subtraction method is known which assigns weights to images obtained with high energy X-rays and low energy X-rays, and subtracts one from the other to extract only bones or calcification diseases, or only soft tissues by erasing bones.
Some known conventional examples in which this dual energy subtraction image taking method is applied to the above described tomographic image taking method include a method of using an imaging plate to take images in two stages; one with high energy and the other with low energy, a method of sandwiching one Cu plate between two imaging plates and obtaining one image with high energy and an image with low energy for each imaging plate simultaneously, as well as a method described in Japanese Patent Registration No. 2871053 of alternating a tube voltage for X-rays irradiated onto an X-ray image taking apparatus, which combines an I.I. and a TV camera, between a high voltage and low voltage for every exposure, or a method of obtaining a high voltage image and low voltage image by replacing X-ray filters to change X-ray energy.
However, the method of performing image taking twice, causes a time interval between two image takings because a high voltage tomographic image is taken first and then a low voltage tomographic image is taken. If the body of the subject moves due to respiration, etc., in this time interval, a position shift occurs between the X-ray images recorded in both taken images, which poses a disadvantage that an accurate difference image cannot be obtained in the dual energy subtraction processing.
The method of using two imaging plates, in which two images with different energy levels can be obtained in one image taking, and so there is no need to take images twice, does not cause an inconvenience due to wobbling of the body of the subject. However, since the two images have not been obtained by switching the tube voltage value of the X-ray tube between high and low, an image produced under the influence of high energy X-rays may appear on the first imaging plate or the second imaging plate may be influenced by low energy X-rays that have passed through the copper plate. Therefore, compared to the method of taking images by switching between tube voltages, the method suffers from a disadvantage that good subtraction images cannot be obtained.
The method of switching the tube voltages or filters alternately for every exposure has the following disadvantages.
A switching time is required, which increases the time required for tomographic image taking and reduces the diagnosis efficiency. Frequent switching of the tube voltage in a short time period increases load on the X-ray tube and reduces its service life. The switching between filters in a short time in synchronization with exposure to X-rays requires an apparatus capable of accurate switching. The same number of calculations as X-ray exposures are required to reconstruct high voltage tomographic images and low voltage tomographic images.